Skip to main content

Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

See other formats


ELECTROLYTIC ANALYSIS OF METALS

213

electrodes and a thermometer, if this is required. The handle of the
rheostat is placed to give the maximum resistance, the current started,
the amperemeter and voltmeter inserted in the circuit, and the rheostat
gradually regulated until the measuring instruments indicate the proper
current and voltage ; the instruments are then cut out.

With the Classen capsule the same directions are to be followed. The
dish containing the electrolyte is placed on the proper stand, the anode
arranged centrally and a few centimetres from the bottom, the cover fitted
and so on, as above.

When rotating electrodes are used, the procedure is the same : when
the electrodes are in place, the rotating apparatus is started so as to give
the prescribed velocity (number of turns per minute), the cover fitted, the
current regulated and the electrolysis continued.

Detection of the End of the Deposition.When the prescribed tune
has elapsed, the deposition of the metal to be determined should be com-
plete. To ascertain if this is so, two means are used : (i) If the deposited
metal is different in colour from platinum (e.g., copper), the level of the
electrolyte is raised a few millimetres by addition of water ; if, after some
time (15 minutes), the newly immersed part of the electrode shows no
coating of the metal being determined, the deposition is complete. (2) A
drop of the electrolyte is removed and tested for the metal by its most
sensitive reactions.

Washing of the Electrodes.With Winlder electrodes, the washing
is very simple. The covers and thermometer are removed and washed
with water, the beaker grasped in the left hand and the supporting stand
removed without interrupting the current. The beaker is then rapidly
but carefully lowered and replaced by a small beaker, which contains,
according to circumstances, distilled water or water acidified with sulphuric
acid and is supported on the stand. After 10-15 minutes the cathode is
detached, washed by a gentle jet of distilled water, then with alcohol and
finally with ether, dried in an oven at 60-70, cooled in a desiccator and
weighed.

With the Classen dish two cases present themselves. If the electrolyte
has no energetic solvent action on the deposited metal, the covers are
removed, the anode detached and placed in a beaker which is kept near,
the dish being emptied into the same beaker and washed with a little water.
The washing is then completed with water, alcohol and ether, and the dish
dried at 60-70, cooled in a desiccator and weighed. If, however, the
electrolyte is acid, it may attack the metallic deposit during these manipu-
lations ; in this case, the washing should be carried out without interrupting
the current. A small stream of distilled water is passed into the dish by
means of a Marriotte's bottle or otherwise, while at the same time the liquid
is drawn off, by a syphon reaching almost to the bottom of the dish, at
such a rate that the level of the surface is always slightly above the deposited
metal. This process is continued until the solution in the dish assumes a
neutral reaction. The dish is then removed from its stand, washed with
water, alcohol and ether, and dried at 60-70.

With rotating electrodes, the same precautions are followed:   at the corresponding with